I understand what I am about to explain is a unique installation but I would greatly appreciate your consideration of my problem.

I have a catamaran sailboat with two engines. I have one Next Step 2 alternator regulator installed. I cannot use the "two alternator" option on the regulator as I am not running two alternators on one engine, but rather two alternators on separate engines. To make this work I have relays installed in the on/off line such that either of the ignition keys can power up the regulator. I have the same setup with relays for the field wire. It has been installed on the boat for twelve years and working well.

However, I just changed battery types so I adjusted the voltage setpoints as per the manual and then paid close attention to what was happening and discovered an interesting problem. When one engine is running the regulator works exactly as set. However, when both engines are running, it transitions to absorption before the voltage setpoint is reached and transitions to float without getting the batteries to the absorption voltage. More detail: abs voltage set to 13.8, float set to 12.9, abs time set to 10 min for lithium ion charging... bulk charge to 13.8 and then off is essentially what they prefer. With setpoints adjusted this way each alternator is controlled appropriately and does as it should. With two engines the amps going in starts off at around 140, but when the voltage sensing at the regulator is only about 13.65 it transitions to abs and then float before the batteries are full.

Each alternator is a BOSCH alternator and puts out about 75 amps at cruising rpm. Thus I charge at about 75 amps with one engine and around 140 or so with two.

The batteries are lithium ion, 1200 amp hour bank and so can accept this volume or more of charging easily.

I have read the manual and trouble shooting guide carefully many times and all wiring is appropriate. I have measured the voltages at the various pins on the regulator with a Fluke true RMS multimeter.

Questions:

1) There are no "bulk voltage" setpoint adjustments or pots. I assume that the regulator stays in "bulk" mode until the voltage setpoint for absorption is reached, and then transitions to absorption mode until the time limit for absorption is met. Correct?

2) Any idea why two engines running would cause the regulator to transition to absorption and float prematurely when it works appropriately with one engine running and how I can correct this issue? I would prefer not to install a whole second regulator if possible as it does work appropriately with one engine running, and I do spend most of my time running just one engine at a time.

I have been monitoring voltage of batteries at the regulator voltage sensing lines and on the meter I have on the batteries while the engine is running and it does NOT get to absorption setpoint. It flicks over at 13.65, not 13.8. I am using a Fluke True RMS meter and a celllog wired directly to the batteries to test. And I have confirmed the programming of set points carefully with Fluke meter.

I do now note a difference in the voltage sensed at the positive sensing line (right at battery pos with no current draw) when using the "ground" pin 8 versus the "alternator neg" pin 3". Voltage is .15 higher when using the alt neg pin then the ground pin. The "ground pin" 8 matches the celllog meter I have wired directly to the batteries so the measurement at the "alternator ground" appears to be artificially slightly high. Which neg is the reference point for the regulator with reference to voltage setpoint transitions? Could I simply increase the setting such that the 0.15 artificial increase is allowed for and the actual battery voltage then gets to 13.8? Ie set the point at 13.95?

I have trialed as you suggested and I am still having trouble. I have reviewed all the wiring and voltage setpoints.

The regulator now shows bulk charge with the LED's but is putting out little or no power to field wire (that LED barely lit) and there is no charge output from the alternator. Then it rapidly switches to showing absorption LED's light sequence and transitions to float. The batteries are about 50% down, and the voltage is 13.2. The abs setpoint is 13.8 and float 12.9.

Previous trouble is now less relevant as it seems to have transitioned to just rapidly going from bulk to float without much charge at all.

Voltages at various pins match exactly as they should for sensing wires and grounds.

I depart for Greenland Monday and would really like to get the engine charging sorted asap